Aspirin vs Clopidogrel

## Overview

Aspirin and clopidogrel are both antiplatelet drugs used for prevention of atherothrombotic events, but through fundamentally different mechanisms. Aspirin irreversibly inhibits COX-1 (blocking thromboxane A2 synthesis); clopidogrel irreversibly blocks the P2Y12 ADP receptor. Their combination (DAPT — dual antiplatelet therapy) is the cornerstone of post-PCI and post-ACS management.

## Mechanism of Action

Aspirin: irreversibly acetylates COX-1 at Ser530, blocking arachidonic acid conversion to thromboxane A2 — a potent platelet activator and vasoconstrictor. Since platelets lack nuclei (cannot regenerate COX-1), the effect lasts the platelet's lifetime (~7–10 days). Low-dose aspirin (81 mg) selectively inhibits platelet COX-1 while largely sparing endothelial COX-1/COX-2 (preserving prostacyclin).

Clopidogrel: a thienopyridine prodrug requiring CYP2C19 activation (active metabolite irreversibly alkylates P2Y12 ADP receptor on platelets). P2Y12 blockade prevents ADP-mediated platelet activation and amplification of thrombus formation. Effect lasts 7–10 days (platelet lifetime).

## Pharmacokinetics

Aspirin: complete GI absorption, deacetylated by plasma esterases, t½ ~20 min but pharmacodynamic effect lasts platelet lifetime. Clopidogrel: prodrug (15% converted to active metabolite by CYP2C19, CYP3A4), t½ active metabolite ~30 min, ~30% of patients have CYP2C19 loss-of-function (CYP2C19*2) leading to reduced activation and pharmacodynamic resistance.

## Pharmacogenomics

CYP2C19 polymorphisms significantly affect clopidogrel efficacy — 25–30% of patients are poor or intermediate metabolizers with reduced platelet inhibition. Genetic testing or platelet function testing can identify these patients. No pharmacogenomic concern for aspirin.

## Clinical Evidence and Combinations

Aspirin monotherapy: primary and secondary prevention (DAPT guidelines), AF stroke prevention (inferior to OAC, but used when OAC not tolerated). Clopidogrel monotherapy (CAPRIE): marginally superior to aspirin for secondary prevention in atherothrombotic patients. DAPT (aspirin + clopidogrel): post-ACS and post-PCI — 12 months is standard.

## GI Toxicity

Aspirin: dose-dependent GI bleeding risk, including gastric ulcers (COX-1 inhibition in gastric mucosa reduces cytoprotective prostaglandins). Clopidogrel: less direct GI mucosal toxicity but still increases GI bleeding risk. PPIs are co-prescribed with DAPT.

## Key Takeaways

- Aspirin: cheap, COX-1 inhibitor, pharmacogenomics-independent, GI mucosal risk
- Clopidogrel: P2Y12 ADP blocker, prodrug (CYP2C19 dependent), no GI mucosal risk
- Both irreversible (7–10 day effect); DAPT combines both mechanisms for maximum antiplatelet effect